CVE-2020-26953 in Firefox
Summary
by MITRE • 12/09/2020
It was possible to cause the browser to enter fullscreen mode without displaying the security UI; thus making it possible to attempt a phishing attack or otherwise confuse the user. This vulnerability affects Firefox < 83, Firefox ESR < 78.5, and Thunderbird < 78.5.
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Analysis
by VulDB Data Team • 02/09/2026
This vulnerability in Mozilla Firefox and Thunderbird represents a significant security flaw that undermines user trust and browser security mechanisms. The issue allows malicious actors to programmatically force browsers into fullscreen mode without proper user consent or security warnings, creating an environment where phishing attacks can be executed with enhanced effectiveness. The vulnerability specifically affects versions prior to Firefox 83 and Firefox ESR 78.5, as well as Thunderbird versions before 78.5, indicating a widespread impact across the Mozilla ecosystem. This flaw operates at the intersection of user interface security and web application behavior, exploiting the browser's fullscreen API implementation to bypass established security protocols.
The technical nature of this vulnerability stems from insufficient validation of fullscreen requests within the browser's security framework. When a web page attempts to enter fullscreen mode, the browser should typically display security UI elements to inform users of the transition and confirm their intent to proceed. However, this vulnerability allows bypassing these safeguards, enabling attackers to force fullscreen mode without user awareness. The flaw likely resides in how the browser handles fullscreen API calls, particularly in scenarios where user interaction is not properly required or validated before transitioning to fullscreen state. This represents a violation of the principle of least privilege and user consent that should govern all security-sensitive browser operations.
The operational impact of this vulnerability extends beyond simple user confusion to create genuine phishing opportunities and user deception scenarios. When users are unexpectedly thrust into fullscreen mode, they lose the ability to easily identify legitimate browser UI elements, making it difficult to distinguish between authentic browser interfaces and maliciously crafted impostors. This creates an ideal environment for social engineering attacks where attackers can exploit the user's lack of awareness about the fullscreen transition. The vulnerability essentially removes a critical layer of user protection that should prevent unauthorized interface changes, potentially allowing attackers to overlay malicious content over legitimate browser windows or redirect users to fake authentication pages.
Security implications of this vulnerability align with several common attack patterns documented in the attack framework, particularly those involving user interface deception and phishing techniques. The flaw enables what is commonly referred to as "UI redressing" or "UI overlay" attacks where malicious actors can manipulate the browser interface to their advantage. From a compliance standpoint, this vulnerability may violate various security standards including those related to user interface security and secure coding practices. The issue also relates to CWE-693, which covers protection mechanism failures, and demonstrates how inadequate implementation of security controls can create exploitable conditions. Organizations using affected browser versions face increased risk of successful phishing campaigns and user deception attacks that could compromise sensitive information.
Mitigation strategies for this vulnerability primarily involve updating to the patched versions of Firefox, Firefox ESR, and Thunderbird as recommended by Mozilla. System administrators should prioritize immediate deployment of security patches across all affected installations to eliminate the risk of exploitation. Additionally, organizations should implement monitoring for suspicious fullscreen API usage patterns in web applications and consider browser security policies that restrict fullscreen functionality for untrusted content. User education regarding the importance of recognizing browser UI elements and understanding fullscreen transitions can also serve as an additional defensive measure. The vulnerability highlights the critical importance of maintaining up-to-date software and demonstrates how seemingly minor interface flaws can create significant security risks when exploited by malicious actors.